1. Field of the Invention
The present invention relates to a laser unit, in particular, a high-power laser unit wherein laser output can be accurately corrected.
2. Description of the Related Art
In a laser unit used for laser processing or the like, it is often necessary that laser output be corrected in order to obtain desired processing performance. For example, Japanese Unexamined Patent Publication (Kokai) No. 2005-251855 discloses a laser unit, wherein a laser oscillator is warmed-up or cooled-down so that a measured temperature of a certain site of the laser oscillator or a laser application unit is within a constant temperature range, and then a correction coefficient for correcting a laser output command value is determined.
Japanese Unexamined Patent Publication (Kokai) No. 9-107146 discloses a laser output control device configured to randomly transmit a plurality of voltage commands at different levels to a laser power supply before actual laser operation, whereby a data table, representing the relationship between a voltage command value or a current command value and an energy value of a laser beam, is generated. In the actual laser operation, the control device extracts the command voltage or command current value from the data table, corresponding to an output command value, and transmits the extracted value to the laser power supply.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-61565 discloses a technique for stabilizing laser output by correcting a deviation in laser energy relative to a target energy value immediately after the initiation of laser oscillation. In the technique, information, relating to laser beam energy and laser excitation intensity of at least the start of each continuous oscillating operation, is stored in a storing means, and the stored information, relating to the laser beam energy and the laser excitation intensity at the start of the previous continuous oscillating operation, is used to determine the laser excitation intensity at the start of the next continuous oscillating operation.
As a method for obtaining laser output corresponding to a command value in the laser unit, a method using feedback control is known. However, the feedback control requires a measurement means, such as a laser power monitor, which can rapidly and correctly measure the laser output. Since laser power monitors are expensive and may be unreliable depending on a wavelength band of the laser, a power monitor having a long measurement cycle may be necessary. In such a case, feedback control with high accuracy cannot be performed. Therefore, in many cases, open-loop control or feedforward control is combined with feedback control so as to obtain a laser output corresponding to a command value. When open-control or feedforward control is combined with feedback control, laser output must be corrected so as to determine a coefficient for previously predicting actual laser output relative to command output.
In addition, since the laser unit is designed to output rated power, a ratio of fluctuation to laser output is considerably large when the laser output is low. Similarly, since the laser power monitor is designed to accurately measure the rated power, a measurement error is relatively large at the low laser power. Further, since the magnitudes of the fluctuation and the measurement error depend on an environmental factor inside or outside the laser unit, it is difficult to accurately correct the laser output.
Furthermore, since the laser and the laser power monitor are affected by various factors, the low laser output and the measurement thereof depend on temperature history before the measurement. In other words, even when a temperature measurement result of some points inside or outside the laser unit and/or the laser power monitor coincides with another temperature measurement result, two measurement results of laser output do not always coincide with each other.